As a color copying machine, a color printer or the like using electrophotography, there is available one in which four photosensitive members are arranged in tandem. A tandem-type image forming apparatus is advantageous in that it allows the use of relatively many types of recording media and that it has high recording speed, and the tandem-type image forming apparatus is becoming mainstream as a recent color image forming apparatus.
As a form of such a tandem-type image forming apparatus, Japanese Patent Application Laid-Open No. 2003-279875 and Japanese patent Application Laid-Open No. H10-221617 disclose apparatuses in which two laser scanner units are provided for four photosensitive members (2-box type). In each of the laser scanner units mounted in this type of image forming apparatus, one polygon mirror is used for two optical systems (common use), thereby achieving a reduction in the size and cost of the image forming apparatus.
In some tandem-type image forming apparatuses, the four photosensitive members are not arranged in a straight line. Japanese Patent Application Laid-Open No. 2001-42595 discloses a construction in which, of the first through fourth photosensitive members, the second and third photosensitive members protrude toward the transfer belt by approximately 1 mm. In the apparatus as disclosed in the above-mentioned publication, in the full color printing mode, the transfer belt for conveying recording sheets is in contact with all the four photosensitive members, whereas, in the monochrome printing mode, the transfer belt is separated from the three photosensitive members other than the photosensitive member for black. Since the second and third photosensitive members protrude toward the transfer belt by approximately 1 mm, the recording sheet carrying surface of the transfer belt is flat at the time of monochrome printing while maintaining an appropriate contact state of the transfer belt with respect to each photosensitive member at the time of full color printing, so that it is possible to prevent the recording sheet from being separated from the transfer belt.
In the apparatus disclosed in this publication, the laser scanner units corresponding to the four photosensitive members constructed of four separate units (4-box type), so it is possible to realize an optically appropriate layout by shifting the second and third laser scanner units, which are of the same construction, in parallel toward the transfer belt by the same distance by which the second and third photosensitive members protrude toward the transfer belt (which, in this case, is 1 mm).
However, when applying a 2-box type laser scanner unit, which is advantageous from the viewpoint of achieving a reduction in size and cost, to an apparatus in which the four photosensitive members are not arranged in a straight line, the following problem is involved.
For example, in the image forming apparatus shown in FIG. 4, a photosensitive drum 300 is provided at a position where its optical path length is larger by 1 mm as compared with that of a photosensitive drum 301 (A−B=1 mm). Further, a photosensitive drum 303 is provided at a position where its optical path length is larger by 0.5 mm as compared with that of a photosensitive drum 302 (301) (C−B=0.5 mm). To maintain a uniform imaging state on each photosensitive drum, imaging lenses 101 and 102 are of the same optical characteristics, whereas imaging lenses 100 and 103 have different optical characteristics. Thus, the two laser scanner units 200 and 201 are of different constructions. This difference in construction makes the advantage of the 2-box type apparatus in terms of cost so much the less.